The present invention relates to an apparatus for producing articles by shaping thermoplastic plastics foil.
Shaping of plastics foils often takes place when packaging with thermoplastic foils. For example, the foil may first be heated in a deep drawing tool by heat transfer from a heating plate. Subsequently, the foil is sucked by a fluid pressure difference into a mould positioned opposite the heating surface, with the mould not being pre-heated. The heated foil is stretched and stabilizes its shape after contacting the cold walls of the shaping mould. The moulding of the material begins at the moment the foil is sucked into the pre-heated mould and ends for the respective parts of the foil as they are cooled by contacting the unheated mould. As the deep-drawn corners and edges of any particular shape are last to contact the walls of the mould, these portions are stretched the most and therefore have the smallest wall thickness of the newly formed deep-drawn container. One example of a container shaped as above stated is shown in FIG. 2 of the accompanying drawings.
From West German Offenlegungsschrift No. 16 04 444 a method is known in which, for the shaping of polypropylene or polyvinyl chloride foils, individual zones of the foil are pre-heated to different degrees. First the foil edge which normally is used for clamping is heated and then the remaining portion is heated by contacting a pre-heated surface. Subsequently the shaping operation itself takes place, with the edge contributing to the extension, with the total extension being somewhat less as a result. Here, too, the problem arises in that the corners and edges of the container are subject to the largest extension forces resulting in the smallest wall thickness.